1. Field of the Invention
The present invention relates to a sensor unit for assay in utilizing attenuated total reflection. More particularly, the present invention relates to a sensor unit for assay in utilizing attenuated total reflection, in which a prism as an optical element can be prevented from scratches, dirt or other damages.
2. Description of the Related Art
An assay apparatus assay in utilizing attenuated total reflection is used for various kinds of studies in a biochemical field or the like, for example to study interaction of protein, DNA and various biomaterials, and to select candidate drugs by screening. Also, the technique is useful in the fields of the clinical medicine, food industries and the like.
A surface plasmon resonance (SPR) sensor is known as an assay apparatus in utilizing attenuated total reflection. A thin film/dielectric interface of a metal film is fitted on a dielectric block. Light is directed to the thin film/dielectric interface in a manner conditioned for total reflection. Surface plasmon is a term to mean the compressional wave created on the surface of the metal and included in plasmon as quantized expression of the compressional wave. Free electrons in a metal vibrate to generate the compressional wave.
In the assay apparatus, the sensing surface is positioned opposite to the interface where the metal thin film is connected with the dielectric block. The sensing surface is caused to create surface plasmon resonance. Reaction of samples is assayed by detecting the SPR on the sensing surface.
Upon the total reflection created on the metal/dielectric interface, a small component of the light passes through the metal film without reflection, and penetrates to the sensing surface. A wave of the penetrating component is called an evanescent wave. Surface plasmon resonance (SPR) is created when frequency of the evanescent wave coincides with that of the surface plasmon. In response to this, intensity of the reflected light attenuates remarkably. In the assay apparatus, the attenuation in the reflected light reflected by the metal/dielectric interface is detected, to recognize creation of the SPR on the sensing surface.
A resonance angle or an angle of incidence of light for creation of surface plasmon resonance depends upon a refractive index of a medium of transmission of evanescent waves and surface plasmon. In other words, a change in the refractive index of the medium of transmission causes a change in the resonance angle of creation of SPR. The substance or sample in contact with the sensing surface is the medium for transmitting the evanescent waves and surface plasmon. When binding, dissociation or other reaction occurs on the sensing surface between two molecules or samples, the resonance angle changes because of a change in the refractive index of the medium of transmission. The SPR assay apparatus finds the changes in the resonance angle, to assay the interaction between the molecules or samples.
An assay apparatus for assay in utilizing attenuated total reflection is used for various kinds of studies in a biochemical field or the like, for example to study interaction of protein, DNA and various biomaterials, and to select candidate drugs by screening. Also, the technique is useful in the fields of the clinical medicine, food industries and the like. A sample or biomaterial, such as protein, is handled as sample fluid for the purpose of preventing deactivation or modification due to drying. The sample fluid contains biomaterial and fluid medium, examples of which include pure water, physiological saline water, liquid buffer and the like.
JP-A 6-167443 disclose an SPR assay apparatus in which an optical system of Kretschmann configuration is used for incidence of light to the metal film. According to the Kretschmann configuration, the thin film/dielectric interface of the metal film is fitted on a prism, which condenses light and directs the light to the thin film/dielectric interface in a manner conditioned for total reflection. A sensing surface is overlaid inside the flow channel, for immobilizing the sample. Ligand fluid is introduced to the flow channel for immobilizing the ligand on the sensing surface. After this, analyte fluid is introduced for contact of the analyte and the ligand, to assay the interaction between those.
U.S. Pat. No. 5,164,589 and U.S. Pat. No. 5,313,264 (corresponding to JP-B 3294605) discloses a sensor of the chip type, which according to the SPR system is placed on the assay stage, the sensor including a glass base board which is dielectric and transparent, and metal film overlaid thereon. A plurality of sensors of the chip type are contained in a case. The case has a first opening for appearance of the sensing surface, and a second opening, positioned opposite to the first opening, for appearance of a back surface reverse to the sensing surface. The sensors are loaded in the assay apparatus removably in a state contained in the case. A prism is disposed on the assay apparatus. An upper surface and the glass substrate is connected by use of the matching oil. After the sensors of the chip type are loaded, the ligand is immobilized on the sensing surface, and then assayed for interaction with an analyte.
However, there is a problem in the above assay by use of the sensor of the chip type. As the sensing surface appears externally, the sensing surface is likely to dry if removed from the assay apparatus after the sample immobilizing flow. This is harmful to keeping characteristics of the ligand, for example, an enzyme characteristic of a protein as ligand is likely to deactivate. The assay must follow the sample immobilizing flow shortly for the purpose of preventing the drying of the sensing surface. The same assay stage is used for the immobilization and the assay process. The assay process would be impossible during the immobilization for next sample. The immobilization has a low speed of a rate-determining level defining the speed of the entire assay system even though the assay process is possible in a quick manner. Also, the necessity of the matching oil for the glass board and the prism causes high complexity of the placement of the sensor unit to the assay apparatus.
In U.S. Pat. Pub. No. 2002/0127706 (corresponding to JP-A 2002-296177) and U.S. Pat. Pub. No. 2003/0184755 (corresponding to JP-A 2003-240705), a sensor unit is disclosed in which a prism is used as a dielectric block, a sensing surface is constituted by an upper surface of the prism. A flow channel block is disposed higher than the sensing surface for flow of samples including a ligand and analyte. The sensor unit supports those elements fastened together. As the flow channel block is positioned at the sensing surface in the sensor unit, it is possible to preserve the sensing surface in a humid state after the sample immobilizing flow. Occurrence of failure due to the drying can be prevented. Also, high through-put can be obtained in operation because the sensor unit after the immobilization can be assayed serially one after another after collective immobilization of a plurality of the sensor unit.
In the sensor unit above having the prism, surfaces of the prism appears externally except for the sensing surface. There is a problem during handling of the sensor unit in occurrence of scratches or dirt outside the prism due to hands or fingers of a person conducting the assay. As the surfaces of the prism are used for entrance and exit of illuminating light, electric noise occurs due to absorption, attenuation or scattering of light caused by scratches or dirt. Precision in the assay is likely to be low. Specifically when the sensor unit includes the prism formed from plastic material, scratches are likely to occur because of the higher softness of the plastic material than glass. If dust sticks on the scratched surface, wiping of the prism may create additional scratches.